Growth fabrics and growth rates of Holocene reefs in the Great Barrier Reef

Over the past five years, scientists of the Bureau of Mineral Resources and James Cook University of North Queensland have drilled 61 holes into 24 reefs throughout the Great Barrier Reef Province between 15°30S and 25°50S. Eleven holes penetrated to the Pleistocene and nearly 250 radiocarbon dates have been recorded. Analysis of drill-hole core has delineated five major biosedimentologic facies - coralline algal facies. coral-head framework facies, branching-coral framework facies, detrital carbonate facies, and detrital siliciclastic facies. Latitudinal uniformity in framework facies contrasts markedly with major regional variations in detrital facies, the reefs of the mid-shelf of the central region being dominated by clastic carbonates. Depositional rates of detrital carbonate facies vary between 1-4 m/1000 yr for sand flat progradation , 7-9 m/1000 yr for trade wind storm sedimentation, and 13-18 m/1000 yr for high-energy low-frequency events. Framework growth rates varv from 1- 16 m/1000yr with low rates (2 m/I000 yr) for coralline algae. intermediate rates (up to 7 ml 1000 yr) for coral head facies, and high rates (up to 16 m/lOOO yr) for branching frameworks. Rates of 8-12 m/1000 yr occur in all environments: modes of 7-8 m/1000 yr typify patch reefs , 4-6 m/1000 yr typify windward margins. and 3-9 m/1000 yr typify leeward margins. Fringing reefs usually grow at rates of 1-4 m/1000 yr and are dominated by coral-head facies. Depth to the Pleistocene is generally greater in the central region compared to the northern and southern reefs: reef initiation, however, began at the same time throughout the province (8-9000 yr B. P.). Initial reef growth lagged significantly behind sea-level rise, such that water depths of up to 12 m developed over reef surfaces prior to sea-level stabilisation. However, some reefs in the southern region exhibit no growth lag - initial colonisation and growth maintaining pace with sea-level rise. The growth rate of most reefs decreased markedly as reef surfaces approached stabilised sea level.

过去五年间，北昆士兰州詹姆斯·库克大学（James Cook University）与矿产资源局（Bureau of Mineral Resources）的科研人员，在纬度介于15°30′S至25°50′S的大堡礁海域范围内，对24座珊瑚礁完成了61个钻孔作业。其中11个钻孔穿透至更新统（Pleistocene）地层，目前已累计记录近250组放射性碳定年数据。对钻孔岩心的分析共划分出5类主要生物沉积相（biosedimentologic facies）：珊瑚藻相（coralline algal facies）、珊瑚礁格架相（coral-head framework facies）、分枝珊瑚格架相（branching-coral framework facies）、碎屑碳酸盐相（detrital carbonate facies）以及硅质碎屑相（detrital siliciclastic facies）。

格架相的纬度分布一致性，与碎屑相显著的区域差异形成鲜明对比：中部陆架区的珊瑚礁以碎屑碳酸盐沉积为主。碎屑碳酸盐相的沉积速率存在显著差异：砂坪加积作用下的沉积速率为1~4米/千年，信风风暴沉积的速率为7~9米/千年，高能低频事件沉积的速率可达13~18米/千年。

格架生长速率介于1~16米/千年：珊瑚藻相的生长速率最低，仅2米/千年；珊瑚礁格架相的生长速率中等，最高可达7米/千年；分枝珊瑚格架相的生长速率最高，可达16米/千年。各类沉积环境中均存在8~12米/千年的生长速率区间：斑礁（patch reefs）的典型生长速率为7~8米/千年，迎风岸（windward margins）的典型生长速率为4~6米/千年，背风岸（leeward margins）的典型生长速率为3~9米/千年。岸礁（fringing reefs）的生长速率通常为1~4米/千年，且以珊瑚礁格架相为主。

与北部和南部珊瑚礁相比，中部区域的更新统埋藏深度普遍更大，但整个大堡礁海域的珊瑚礁均于同期开始发育（距今8000~9000年）。珊瑚礁初始生长阶段显著滞后于海平面上升过程，因此在海平面稳定前，礁体表面的最大水深可达12米。但南部区域的部分珊瑚礁并未出现生长滞后现象，其初始定植与生长过程始终与海平面上升速率保持同步。当礁体表面趋近稳定海平面时，多数珊瑚礁的生长速率会显著下降。